1. Field of the Invention
The present invention relates to an apparatus that monitors tire pressure. More particularly, the present invention relates to an apparatus that remotely excites a device in a tire, detects data from the device, and processes and displays the data as a tire pressure of the tire to a vehicle operator.
2. Description of the Related Art
On board tire pressure sensing devices are known in the art. The tire pressure sensing devices are a convenience to vehicle operators and also a safety feature that has been mandated by the National Highway Traffic Safety Administration (41 CFR Part 571) for reducing the incidence of accidents caused by under-inflated tires. In the final ruling issued by NHTSA, two basic systems for tire pressure monitoring were described. More particularly, direct and indirect pressure measurement devices were described.
The indirect tire pressure monitoring systems are found through the rotational speeds as measured by the vehicle's anti-lock braking system (ABS). Although this system has the advantage of using equipment already installed on many vehicles, there are at least two major problems. First, sensitivity is a problem. Second, the indirect tire pressure monitoring systems may not identify critical combinations of multiple under-inflated tires. Since the indirect tire pressure monitoring systems operate by comparing the rotation speeds of individual tires, if a vehicle's tires are uniformly under-inflated, the system will not detect any rotational difference.
Direct tire pressure monitoring systems have sensitivity and an ability to resolve the inflation situation in each respective tire. A complication exists in accessing the information from within the rotating body of the tire. A direct electrical connection to any device becomes impractical because the tire rotates. The tire is also in an aggressive environment and any tire pressure monitoring system needs to be easily removable from the vehicle. Consequently, a practical approach for directly monitoring tire pressure is to use some sort of non-contact sensor device. An electronic pressure sensor coupled to a battery-powered radio frequency transmitter is known in the art. Tire pressure is relayed to the operator through a central receiver that identifies each tire and a pressurization state of the tire.
The main advantages of such direct tire pressure monitoring systems are increased sensitivity, an elimination of confounding variables present in the indirect tire pressuring monitoring system, and an ability to measure pressure in a stationary vehicle.
The direct tire pressure monitoring systems also suffer from a number of drawbacks. The direct tire pressure monitoring system is complex and expensive. The direct tire pressure monitoring system is also heavy and this device weight could potentially affect tire balance itself. Further, direct tire pressure monitoring systems have negative issues with regard to a battery life and battery disposal.
Remotely queried sensor systems unrelated to monitoring tire pressure are known in the art. An example of one such passive device is an electronic article surveillance (EAS) marker. The marker has a resonant circuit created by an antenna and a diode. Alternatively, the resonant circuit has the antenna with a capacitor combination. When interrogated by an alternating electromagnetic field, the circuit resonates. Thereafter, the circuit generates harmonics of the incident field. The receiving antenna detects either the generated harmonics or a depletion of an incident field. However, there are problems with such a system as the broad bandwidth and low amplitude of the harmonics makes these markers difficult to detect reliably.
In another example of the electronic article surveillance marker, the marker has a high magnetic permeability element. The high magnetic permeability element is placed adjacent to an element of higher magnetic coercivity. The high magnetic permeability element being adjacent to the element of higher magnetic coercivity resonates when interrogated by an alternating electromagnetic field due to nonlinear magnetic properties. The high magnetic permeability element adjacent to the element of higher magnetic coercivity generates harmonics of the interrogating frequency that are detected by a receiving coil.
For these electronic article surveillance markers, harmonics detection is difficult because of low amplitude. It is also complicated by the presence of other nonlinear ferromagnetic objects within the interrogation area, such as for example articles of magnetic recording material.
U.S. Pat. Nos. 4,510,489 and 4,510,490 to Anderson, III, et al., (hereinafter collectively as “Anderson”) disclose magneto-mechanical electronic article surveillance. The marker has a thin strip of magnetostrictive ferromagnetic material. The magnetostrictive ferromagnetic material is placed adjacent to a magnetic element of higher coercivity (hereinafter “a magnetically hard element”). A non-alternating magnetic bias is placed on the magnetostrictive ferromagnetic material, and causes a mechanical strain in the magnetostrictive ferromagnetic material. This strain affects a resonant frequency of the magnetostrictive ferromagnetic material. The device is designed with appropriate dimensions and magnetic properties to mechanically resonate at a predetermined frequency when interrogated by an incident alternating magnetic field.
The resonance of the magnetostrictive ferromagnetic material can be detected electro-magnetically. The magneto-mechanical electronic article surveillance marker thus has advantages over previous electromagnetic markers of high sensitivity, high operating reliability and low manufacturing cost.
Magnetostriction is a property of a ferromagnetic material that changes volume when subjected to a magnetic field. When biased by a non-alternating magnetic field, magnetostrictive material stores energy via mechanical strain. This storage, affects the Young's modulus, E, of the material. Such magnetostrictive materials can be caused to resonate in an alternating magnetic field. The fundamental resonant frequency, FR, of a magnetostrictive ribbon can be described as a function F:FR=F(L,E,ρ,σ)where L is the ribbon length, ρ is a density, and σ is the Poisson ratio.
The relationship between the biasing non-alternating magnetic field strength, Young's modulus and the resonant frequency attained by the magnetostrictive strip is more complicated. Variations in the biasing magnetic field strength shift a frequency at which a maximum amplitude response is created (the resonant frequency). The system's resonant frequency can be designed by varying a geometry, one or more mechanical properties of the magnetostrictive material, and a strength of the biasing non-alternating magnetic field. Mechanical processes such as annealing can further manipulate one or more mechanical properties thereof.
U.S. Pat. No. 5,628,840 to Hasegawa discloses a composition of a magnetostrictive material with relatively linear magnetic behavior. Hasegawa further discloses a response of the composition in a resonant frequency versus a bias field. This magnetostrictive material has an advantage in magneto-mechanical electronic article surveillance markers of providing a relatively strong signal for harmonic detection.
A number of vibrations of the magnetostrictive element can be damped by a mechanical interaction. Therefore, the device preferably has the magnetostrictive element in a chamber and capable of movement. Also, the biasing magnet preferably is optimal so as not to attract the magnetostrictive element to impede free movement. U.S. Pat. No. 5,499,015 to Winkler, et al. (hereinafter “Winkler”) discloses a resonant chamber. The resonant chamber is in a retail product or package.
U.S. Pat. No. 6,393,921 B1 to Grimes, et al. discloses a number of magnetostrictive materials in an assembly. The assembly measures pressure remotely, without any direct electrical hardwire connection. The assembly has a magnetostrictive strip that is held adjacent to a diaphragm. A magnetically hard element is connected to the diaphragm. As pressure changes, a deflection of the diaphragm occurs. This deflection changes the proximity of the magnetically hard element relative to the magnetostrictive element. The non-alternating magnetic bias on the magnetostrictive element changes which results in a change of the magneto-mechanical resonant frequency when subjected to an alternating magnetic field. The resonance can be remotely sensed by electromagnetic devices.
Grimes discloses an embodiment where the magnetostrictive material is in a pressure sensor. The pressure sensor has the magnetostrictive materials and the magnetically hard element in a defined proximity to each other to provide a constant non-alternating magnetic biasing field applied to the magnetostrictive element. Grimes further discloses that the magnetostrictive element has a mechanically hardened region.
A density change in a gas surrounding the pressure sensor is associated with a pressure change. This pressure change thus causes a shift in resonant frequency. In both disclosed embodiments, the interrogating signal and receiver scans a number of frequencies. This scanning locates a resonant peak, and relates a frequency to a pressure datum. Also disclosed is a method which excites with an impulse and uses a fast Fourier transform (FFT) to find a number of resonant peaks.
However, there are known problems associated with such a pressure sensor. The magnetostrictive response is temperature sensitive, primarily due to a dependence on Young's modulus. Consequently, the pressure sensor of Grimes requires independent temperature correction. For the purpose of determining a thermal drift of the pressure sensor, a correcting temperature measurement can be made with another second test device similar to the pressure sensor, that is not exposed to any varying non-alternating bias field strength or that is not exposed to any changing gas density, and that is in the same thermal environment as the pressure sensor.
Accordingly, there is a need for a tire pressure monitoring system that eliminates one or more of the aforementioned drawbacks and deficiencies of the prior art.